The National Crime Agency said in a recent report it was yet to understand if there was a demand from drug users for the substance, or whether they were even aware they were taking it with their usual heroin hit.

Kirsten Horsburgh, the national Naloxone Coordinator at the Scottish Drugs Forum, said the advice for dealing with fentanyl-related overdoses remains the same as for other opiates.

But she says that more doses may be needed to save someone who has taken fentanyl.

She said: "You never really know what is in your heroin.

"It is frequently different strengths, so at times there are additional substances added to make it stronger or of a higher purity.

"People should always be vigilant if they are injecting heroin, particularly if they have not been using it for a while."

If fentanyl was to be present in heroin, the risks of overdose would be higher.

"People need to be aware of the signs and symptoms of overdoses, that is the most crucial part.

The death of rock star Prince has been attributed to fentanyl and last year an image of a Canadian mother cradling her dying son after he took the drug went viral after she posted it online.